1. Field of the Invention
The present invention relates to a secondary battery (nonaqueous secondary battery) including a nonaqueous electrolytic solution.
2. Description of Related Art
A nonaqueous secondary battery such as a lithium ion secondary battery has a small size, a light weight, and high energy density and thus is superior in output density. Therefore, a nonaqueous secondary battery has been preferably used as a so-called, portable power supply for a PC, a portable device, or the like or as a drive power supply for a vehicle.
Typically, such a battery is used in a state where the voltage is controlled to be in a predetermined region (for example, 3.0 V to 4.2 V). However, even when a higher current than usual is supplied to the battery due to malfunction or the like, the battery may be overcharged to higher than a predetermined voltage. During overcharge, gas may be produced by an electrolytic solution being decomposed, and the internal temperature of the battery may be increased by an active material generating heat. Therefore, as a safety mechanism for stopping the progress of overcharge, a pressure-operated current interrupt device (CID), which interrupts a charging current when the internal pressure of a battery case is a predetermined value or higher, is widely used.
Examples of the prior art documents relating to the CID include Japanese Patent Application Publication No. 2013-175309 (JP 2013-175309 A), Japanese Patent Application Publication No. 2013-157154, and Japanese Patent Application Publication No. 2013-196798. For example, JP 2013-175309 A discloses a nonaqueous secondary battery including a CID, in which an electrolytic solution contains a compound (hereinafter, also referred to as “gas producing agent”) that produces gas at a predetermined battery voltage or higher, and a conductive material layer having a porosity of 35% to 55%, which is a reaction field of the gas producing agent, is provided between a positive electrode and a separator. According to this configuration, during overcharge, the gas producing agent can be caused to stably react (be decomposed) so as to produce gas. As a result, the internal pressure of a battery case can be rapidly improved, and the CID can be accurately operated.
According to the finding of the present inventors, the amount of gas required to operate a CID during overcharge (hereinafter, also referred to simply as “overcharge gas amount”) may vary depending on the size of the remaining space in a battery case (that is, a volume obtained by subtracting the volume of an electrode body, an electrolytic solution, and auxiliary components such as a terminal from the internal volume of the battery case). For example, it is considered, under the same various conditions such as the setting of an operating pressure and the content of an electrolytic solution, the lower the remaining space volume in a battery case, the less the overcharge gas amount. However, when the remaining space in a battery case is excessively small, a CID is likely to malfunction, which is contradictory. That is, for example, during battery construction or normal use of a battery, an electrolytic solution or the like may be slightly decomposed so as to unexpectedly produce gas. As a result, a CID may malfunction.
In particular, it is required for a nonaqueous secondary battery having a high volume capacity density, which is used for a vehicle, to have a high level of both the battery capacity and the operating accuracy of a CID at the same time. That is, in the battery having a high volume capacity density, in order to increase the battery capacity per unit volume, it is necessary that the remaining space in a battery case be further decreased. Further, as the volume capacity density (in particular, the volume capacity density and the capacity) of a battery increases, it is necessary that, during overcharge, the progress of overcharge be accurately stopped by rapidly operating a CID. On the other hand, even when a battery for a vehicle is exposed to an environment such a high-temperature environment (for example, under a blazing sun) where an electrolytic solution or the like is likely to be decomposed, it is necessary to stop the malfunction of a CID at a high level.